Various dosage forms have been proposed for drugs required to be administered for a prolonged period. Among them, Japanese published unexamined patent application (Toku-Kai Sho) 57-118512 and its corresponding EP-A-0052510 disclose preparation of microcapsules by a phase separation method using a coacervation agent such as a mineral oil or a vegetable oil. Toku-Kai Sho 60-100516 (the corresponding U.S. Pat. Nos. 4,652,441 and 4,711,782), 62-201816 (the corresponding EP-A-0190833) and 63-41416 disclose methods of preparing microcapsules by means of in-water drying. According to these methods, drugs can be efficiently incorporated into microcapsules to give desirable microcapsules with less initial release.
In the case of administering a drug in the form of microcapsules, requirements for microcapsules having high dependency on interaction with functions of the living body are diversified into a variety of phases. Since the matter is concerned with medicines, microcapsules capable of satisfying those various requirements as far as possible have been desired.
There are many reports on microcapsules comprising a water-soluble drug using a biodegradable polymer. However, in the case of using a water-soluble drug, especially a physiologically active peptide having a relatively large molecular weight, the diffusion of the drug thus encapsulated into the polymer is low, and, therefore, the drug is not released at the initial stage until the decomposition or impregnating of the polymer proceeds. Also, as an adverse effect, a large burst at the initial stage cannot be avoided depending on the method of preparation. Thus it is often the case that practical difficulties occur with use as medicines. Especially, in sustained release pharmaceutical compositions over an extended period of time, constant release of the drug with higher accuracy is an important requirement, but no microcapsules satisfying those requirements have been known.